Key result
Atrial pacing at a faster rate resolved a wide QRS interval, suggesting bradycardia-dependent phase 4 block as the cause of the patient's intraventricular conduction delay.
Case Report (n=1)
Bradycardia-dependent phase 4 block should be considered as a potential cause of intraventricular conduction delay when atrial pacing at a faster rate resolves a wide QRS interval.
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May warrant considering phase 4 block in unexplained conduction delay; case report leaves open prospective validation before changing pacing practice.
Cardiac resynchronization therapy device implantation is complicated by a significant rate of failure to place a left ventricular lead via the coronary sinus. The present case describes one such failure ironically leading to resynchronization. The patient's QRS narrowing subsequent to postimplant atrial pacing alone suggests that bradycardia-dependent phase 4 block was the indirect but exclusive cause of the patient's intraventricular conduction delay. Thus, phase 4 block should be considered when atrial pacing at a faster rate resolves a wide QRS interval.
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Thurber et al. (2018) conducted a case report in Intraventricular conduction delay (n=1). Atrial pacing was evaluated on QRS narrowing. Atrial pacing at a faster rate resolved a wide QRS interval, suggesting bradycardia-dependent phase 4 block as the cause of the patient's intraventricular conduction delay.
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